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Multiphase Power Solutions For High-Current, Fast-Transient, Noise-Sensitive Applications

Multiphase Power Solutions For High-Current, Fast-Transient, Noise-Sensitive Applications

March 26 , 2024 0

An important performance indicator of a power supply is its recovery time, which is the time it takes for the output voltage to return to its rated value when a load transient occurs. There is a limit to the recovery time of each power supply, which is related to the bandwidth of the control loop. The larger bandwidth of the control loop means that the inductive current used to compensate for the charge change on the output capacitor during the transient can rise/fall faster, thus restoring the output voltage in a shorter time.
As an example, the four-phase LT8627SP is used to power a 1.8VOUT RF digital load with load currents up to 50A, as shown in Figure 1. The design output voltage of the power supply is 1.8V and the switching frequency is 2MHz. In order to shorten the charge compensation time during the transient, the design does not use high ESR polymer capacitors and electrolytic capacitors, but uses low ESR ceramic capacitors. Interleaved PWM technology (90° per phase) is also applied to increase the equivalent ripple frequency, thereby increasing the control bandwidth.

The purpose of adjusting the compensation network is to achieve a phase margin of at least 45° and a gain margin greater than 8dB while maximizing the bandwidth. Finally, the maximum bandwidth of the control loop is adjusted to 280kHz, with a phase margin of 45° and a gain margin of 9dB, as shown in the Bode chart in Figure 2. For comparison, the single-phase LT8627SP was tested for 1.8V, 12A output with equivalent output capacitance per phase of (2×100µF+1× 1µf +1×0.1µF). Its Bode diagram with the same stability criteria is also shown in Figure 2.

To compare test recovery times, both the four-phase and single-phase LT8627SP were tested for 50% load transients with a piezoswing rate of 6A/µs per phase. The results (see Figure 3) show that the recovery time of the four-phase transient rising edge is about 2.5μs. This is almost ten times shorter than the single-phase LT8627SP shown in Figure 4.

Minimize Transient VPP For High-Current Wireless Applications

The multiphase operation of the Silent Switcher 3 architecture has been widely used in many customer power supply designs. Figure 5 shows another example where the LT 8627SP is powering a fast, high-current transient SoC for wireless applications with a voltage of 0.8VOUT and a transient load of 22 to 60A in 1µs. To prevent transient degradation of SoC performance, a VPP of less than 5%(40mV) is required.

As mentioned earlier, for the four-phase interwoven LT8627SP, you can expect a fairly high control bandwidth of around 300kHz. In the time domain, the relationship between voltage deviation and control bandwidth during load transients can be roughly modeled by the following formula:

Thus, when the ripple voltage is 10mV, a minimum output capacitance of 1583µF can be obtained. The capacitance value chosen at design time should be higher than this value and different from before, because more polymer capacitors are used to provide adequate damping during transients. Since the output capacitance also affects the loop bandwidth and stability, the final output capacitance is determined through repeated optimization.

The four-phase LT8627SP is interwoven with a switching frequency of 1MHz to form a ripple frequency of 4MHz. After the minimum output capacitance was determined, a VPP of 35mV(4.4%) was achieved at a voltage swing rate of 28A/µs over load transients of 22 to 50A. The transient waveform is shown in Figure 6. In order to verify the stability of the control loop, Baud chart measurements were carried out with 50A load, and the results were shown in Figure 7. At 50A, the bandwidth of the control loop is 322kHz and the phase margin is 50°.

In other performance tests, efficiency and full-load thermal performance were also measured. As shown in Figure 8, efficiency at loads up to 60A was tested at 12VIN, 0.8VOUT. Including auxiliary losses, the converter achieved peak efficiency of 89% at 25A load and 84% at 60A load.

The thermal imaging in Figure 9 shows the thermal performance of this four-phase design. At 60A load, the highest IC temperature is 66 ° C, while the lowest IC temperature is 61.6 ° C. The maximum temperature deviation between the four ics is about 5 ° C, which indicates that the current sharing between the phases is very balanced.

Design Considerations And Guidelines For The Multiphase LT8627SP

As a peak current mode control IC, the LT8627SP can be easily configured for polyphase operation. In the design need to pay special attention to the following points:
For proper shunt, the VC pins of each IC should be connected together, as shown in Figure 1.

For the four-phase LT8627SP to interweave evenly, the CLKOUT of each IC is configured with a 90° phase shift and fed into the SYNC pin of the next IC. In this configuration, the switching node waveform of each IC is shown in Figure 10. Interlacing is one of the biggest advantages of polyphase depressurization. Because the uniformly interwoven phase multiplies the ripple frequency of the output voltage, the output capacitance value can be significantly reduced. The higher cross-weave ripple frequency also helps to protect the control loop from ripple noise at higher bandwidths. The LT8627SP operates at a switching frequency of up to 4MHz and is available in three phase-shift clock configurations: 180°, 120° and 90°. This means that it can achieve up to 12 phase interlacing without additional components.

For correct voltage detection, the OUTS pins of each IC should be connected together. It is important to note that since all error amplifiers (EA) participate in the control loop, Bode chart injection needs to involve all EA. Therefore, both the detection point (output voltage) and the OUTS pin side need to be connected together to ensure that the disturbance of each EA can be observed evenly.

The RT pin requires a resistor to set the frequency. The resistance value of the main IC should be set to the corresponding desired switching frequency, and the resistance value of the IC should be set to the corresponding frequency 20% lower than the main IC.